Not sure it will work or not. You can try modifying the WCFTestClient.exe.config (You can find this file - C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE depends on your VS installation) and add the following lines
<system.net>
<defaultProxy useDefaultCredentials="true" />
</system.net>
Please make sure you have a backup of the config file. Hope it works.